1. Field of the Invention:
This invention relates to a pulse generator producing pulses having a width free from a variation of power voltage and a threshold voltage of an inverter used therein.
2. Description of the Related Art Including Information Disclosed Under .sctn.1.97-1.99
Pulse generators in the prior art using a CR time constant circuit were widely employed in MOS integrated circuits. Such pulse generators has a series connection of a resistor and a capacitor supplied with a power voltage. An electrical path of source and drain of an MOS FET (MOS Field Effect Transistor) is connected in parallel with the capacitor. A cascade connection of an inverter and a monostable multivibrator is connected between a common connection point of the resistor, the capacitor and the drain of the MOS FET and the gate of the MOS FET.
The capacitor is charged through the resistor, when a voltage at the common connection point rises to the threshold voltage of the inverter, the inverter changes its output to trigger the monostable multivibrator. The multivibrator produces one control pulse by which the MOS FET turns on to discharge the capacitor. After the termination of the control pulse, the charging into the capacitor through the resistor resumes. In this way, the charging and discharging of the capacitor are repeated. Pulses are derived from the output of the monostable multivibrator. A repetition of the derived pulses is controlled by the time constant of the series connection of the resistor and the capacitor.
The repetition of the derived pulses will now be explained in more detail. One repetition cycle consists of a first period of the control pulse and a second period of the charging time. The signal levels at the first and second time periods are opposite to each other. The first time period is determined by the circuit construction of the monostable multivibrator. The second time period depends on the charging current into the capacitor and the threshold voltage of the inverter. The charging current changes by the power voltage. Therefore, the interval of the derived pulses, and more particularly the second time period, is affected from variations of the power voltage and the threshold voltage of the inverter.
As above-explained, the pulse generator in the prior art has a drawback which the interval of output pulses changes with a power voltage and a threshold voltage of an inverter used therein. The threshold voltage of an inverter is easily affected from an operating temperature and a manufacturing condition. Thus, the pulse generator in the prior art cannot produce a stable oscillation.